1 Technical Field of the Invention
The present invention relates generally to electric generators. More particularly, the invention relates to a tandem alternator for a vehicle, which has a high efficiency and includes a voltage regulator that is optimally arranged in the alternator.
2 Description of the Related Art
Japanese Patent First Publication No. H11-98789 discloses a tandem alternator for a vehicle, which includes a single rotary shaft and a front-side and a rear-side power generation unit that are arranged in tandem in the axial direction of the rotary shaft.
The tandem alternator further includes a first and a second rectifier that respectively rectify AC powers output from the front-side and rear-side power generation units into DC outputs. The first and second rectifiers each generate a large amount of heat during operation of the alternator. To effectively dissipate the heat and minimize the distances from the rectifiers to the corresponding power generation units, the first and second rectifiers are respectively arranged at a front and a rear portion of the alternator.
Furthermore, in the tandem alternator, the rear-side power generation unit includes a typical lundell-type field. This field is supplied with field current through a slip ring-brush mechanism that is provided around a rear end portion of the rotary shaft. On the other hand, in order to reduce the axial length of the alternator, the front-side power generation unit employs a lundell-type field that includes a stationary field winding, thereby being able to receive field current without a slip ring-brush mechanism.
However, the use of such a lundell-type field with a stationary field winding requires a greater field current and thus decreases the efficiency of the tandem alternator. Accordingly, it is desirable for the front-side power generation unit to employ a typical lundell-type field as well.
Further, to reduce the axial length of the alternator, it is desirable for the fields of the front-side and rear-side power generation units to be supplied with a field current through a common slip ring-brush mechanism that is provided around the rear end portion of the rotary shaft. More specifically, the slip ring-brush mechanism may include three pairs of slip rings and brushes. The first pair may be electrically connected to the field of the front-side power generation unit; the second pair may be electrically connected to the field of the rear-side power generation unit; the third pair may be grounded and commonly used by the front-side and rear-side power generation units.
Furthermore, to meet different voltage requirements of electrical loads provided on the vehicle, it is preferable that the rear-side power generation unit produces a typical voltage of 12V while the front-side one produces a higher voltage of 42 V. In other words, the output voltage of the front-side power generation unit is preferably higher than that of the rear-side one. This is because the rear-side power generation unit is closer to the vehicle engine and thus subject to a higher ambient temperature than the front-side power generation unit.
Moreover, it is desirable for the tandem alternator to include a single voltage regulator (i.e., controller) that regulates both the output voltages of the front-side and rear-side power generation units. Further, in terms of efficiency, it is preferable for the voltage regulator to receive field current from the first rectifier, rather than the second rectifier, and provide the received field current to the fields of the front-side and rear-side power generation units.
However, when the voltage regulator is electrically connected between the first rectifier and the slip ring-brush mechanism and located in close vicinity to the first rectifier, the distance between the voltage regulator and the slip ring-brush mechanism is accordingly long.
Consequently, as to be described in detail later, the resistance loss of wires that electrically connect the voltage regulator to the slip-brush mechanism will increase, thus decreasing the efficiency of the tandem alternator.
In addition, one may consider decreasing the resistance loss of the wires by increasing the thickness thereof. However, the use of thicker wires will increase both the weight and cost of the alternator and make it difficult to suitably arrange those wires in the alternator.